1. Field of the Invention
The present invention relates generally to nanotechnology and, more particularly, to a method for synthesizing bimetallic nanostructures.
2. Description of the Related Art
One-dimensional (1-D) metallic nanostructures provide unique structure-dependent optical, electrical and thermal properties. In addition, metallic nanostructures are effective electrocatalysts for Oxygen Reduction Reactions (ORR) and alcohol electro-oxidation reactions in Polymer Electrolyte Membrane Fuel Cells (PEMFCs). Conventional PEMFCs, such as nanoparticulate platinum based catalysts, suffer from low efficiencies as well as high cost. Low efficiency of PEMFCs arises from slow oxygen reduction kinetics, resulting in cathodic overpotential. Platinum nanoparticle catalysts possess a relatively high number of defect sites and low-coordination atoms at their surface as a result of a zero-dimensional (0-D) structure, which renders the platinum nanoparticles less active toward ORR and necessitates high loadings in a range of 0.15 to 0.25 mg/cm2 to achieve practical efficiencies.
Koenigsmann et al., in Size-Dependent Enhancement of Electro catalytic Performance in Relatively Defect-Free, Processed Ultrathin Platinum Nanowires, Nano. Lett. 2010, 10, 2806-2811, investigate size dependence of 1-D platinum nanostructures on activity, comparing relevant activity of nanotubes with diameters of 200 nm to that of 1 nm diameter platinum nanowires. Electrochemically determined specific activities for ORR indicate a nearly 4-fold increase in specific activity from 0.38 to 1.45 mA/cm2 as the 1-D platinum nanostructure diameter decreases from 200 nm to 1.3 nm. This size-dependent increase in activity of 1-D nanostructures, as the diameter decreases from the submicrometer range, i.e., 100 nm<diameter<1 μm, to the nanometer range, i.e. diameter<100 nm, contrasts with that of 0-D carbon supported platinum nanoparticles. In 0-D carbon supported platinum nanoparticle catalysts, activity decreases significantly as particle size decreases from the submicrometer to nanometer sizes, particularly when particle size decreases below 5 nm. Nanometer-sized platinum 1-D catalysts activity is observed to arise from contraction of the platinum nanostructure surface. The small diameter of the nanometer platinum nanowire catalysts minimizes precious metal wasted in the core of the nanowire, while also providing increased electrochemical activity.
Nevertheless, a continuing challenge in exploration of size-dependent trends with 1-D nanostructures is the development of environmentally friendly methods for synthesis of crystalline, high purity nanostructures with high aspect ratios and predictable dimensions. Many solution-based methods for preparing 1-D noble metal nanowires have been reviewed by Tiano et al., in Solution-Based Synthetic Strategies for One-Dimensional Metal-Containing Nanostructures, Chem. Comm. 2010, 46, 8093-8130. For example, Xia et al., in Shape-Controlled Synthesis of Metal Nanostructures: The Case of Palladium Adv. Mater. 2007, 19, 3385-3391, provide methods utilizing elevated temperatures and pressures for preparation of anisotropic nanostructures of palladium such as nanorods, nanoplates, nanocubes, and twinned nanoparticles, where control of reaction kinetics with additives, such as inorganic salts and surfactants, yield nanostructures with predictable morphology. Zheng et al., in One-Pot, High-Yield Synthesis of 5-Fold Twinned Pd Nanowires and Nanorods, J. Am. Chem. Soc. 2009, 131, 4602-4603, demonstrate generation of high-quality palladium nanowires and nanorods with diameters of 9.0 nm at elevated temperatures, employing poly(vinylpyrrolidone) as both a surfactant and as an in situ reducing agent.
Although the methods described above generate high quality 1-D nanostructures, a limitation of these synthetic methods is a lack of control over diameter and aspect ratio of the synthesized nanostructures. In addition, surfactant molecules serving as capping agents in these synthetic methods are adsorbed onto surfaces of the nanostructures. Surfactant adsorption limits application of the nanostructures as catalysts, sensors and electrocatalysts, since decreased exposure of the surfaces of the nanostructures inhibits activity.
In light of these limitations, porous template-based methods are employed in synthesis of 1-D nanostructures. Specifically, dimensions of pores within a porous template determine size and morphology of nanostructures grown within the porous template. Regarding template-based synthesis of nanostructured metals, Wang et al., in Pd Nanowire Arrays as Electrocatalysts for Ethanol Electrooxidation Electrochem. Comm. 2007, 9, 1212-1216, provide a method for obtaining 1-D nanostructures through electro-deposition of precursors within either Polycarbonate (PC) or Anodic Alumina Oxide (AAO) porous templates. For example, arrays of palladium nanostructures with uniform diameters of 80 nm were prepared by Wang et al. through electro-deposition within an AAO template having pore sizes of 80 nm. However, the electro-deposition method described by Wang et al. requires additional electrochemical equipment and uses caustic reaction media. Kline et al., in Template-Grown Metal Nanowires, Inorg. Chem. 2006, 45, 7555-7565, describe conventional electro-deposition methods requiring physical vapor deposition techniques to deposit a conductive metallic backing onto porous templates prior to nanostructure deposition. Collectively, these processes are costly, inefficient, and difficult to scale up.
Patete et al., in Viable Methodologies for the Synthesis of High-Quality Nanostructures, Green Chem. 2011, 13, 482-519, describe use of a U-tube double diffusion vessel as both an effective and green method for the production of high-quality 1-D metallic nanostructures under ambient conditions. U.S. Pat. No. 7,575,735 to Wong et al., which is incorporated herein by reference, utilizes a U-tube double diffusion vessel in synthesis of metal oxide and metal fluoride nanostructures. Further, U.S. Patent Publication No. 2010/0278720 A1 to Wong et al., which is incorporated herein by reference, utilizes the U-tube double diffusion vessel to synthesize metal oxide nanostructures. The U-tube methods of Patete et al. and Wong et al. provide metal oxide and metal fluoride nanowires by precipitation of a metal cation with an appropriate anion, i.e., OH− or F−, for growth of the nanowire. However, Patete et al. and Wong et al. do not provide a method to prepare nanowires composed of metal only without other non-metal components, since two separate reagents must react to form the nanowire. Another shortcoming of Patete et al. and Wong et al. is that the metal component within the metal oxide or metal fluoride nanowire maintains a cationic state and is not fully reduced, which reduces catalytic performance of the nanowire, particularly towards ORR. Conventional methods fail to disclose formation of metallic nanowires without non-metal components under ambient, surfactantless conditions.